From the moment that Aaron Swain got out of his tanker truck, which was loaded Monday with hazardous chemicals, and spotted them dripping onto the Capital Beltway, until eight hours later when about 630 nearby residents returned to the homes from which they had been evacuated, the leak's biggest threat hung in the air: the tanker wall might burst.

If it did, fire officials said, the liquid chemicals, all known to be corrosive and capable of burning lungs and skin, would likely spread over a wide area, vaporize, and drift over residential areas, harming many who breathed them.

"There is no guaranteed behavior" of hazardous chemicals, said John P. Kimball, captain of the Fairfax County Fire Department's hazardous materials response team, "and that's what makes them so dangerous."

The tanker's leaking chemicals, which had been used to clean Navy vessels, might have corroded much of the tank shell and burst through it, Kimball said. It was that fear, he said, that sent 100 firefighters to initiate precautionary measures that included evacuating an entire neighborhood south of the spill site and training on the tanker fire hoses capable of spouting 15,000 gallons of water a minute.

Less than 10 minutes after Swain alerted authorities, Kimball's specially equipped team was on the scene trying to determine which chemicals were inside the truck and what was necessary to halt the leak.

"There were some fumes in the area, but they were localized," Kimball said. Firefighters wearing masked yellow rubber suits, which cost more than $1,100 and weigh more than 100 pounds each, were the only persons permitted near the fumes.

After it was discovered that the leak could not be plugged, Kimball said, a replacement tanker was called. Not until every drop of the chemicals was transferred into the second tanker was the threat of the tanker breaking open considered over.